Compared with conventional steel spring systems, air suspension systems have significant advantages and are therefore increasingly being used both on commercial vehicles, such as trucks and buses, and on heavy, passenger vehicles, such as luxury limousines and SUVs. Thus, an air suspension system allows level control independently of load since the current load state can, in each case, be compensated for by adapting the bellows pressure in the spring bellows of the air springs. By virtue of the progressive spring characteristics of the air springs, an air suspension system likewise offers particularly reliable road contact for the wheels and a comfortable response during outward and inward deflection of the wheels. Another advantage of air suspension systems is that the ground clearance of the vehicles can be modified if required, e.g., increased for off-road use and reduced for high-speed travel on freeways.
In the case of commercial vehicles, there is also the fact that the vehicle body can be lowered or adjusted to a suitable height for loading and unloading. Thus, for example, the vehicle chassis of an air-sprung truck or trailer can be lowered to set down an interchangeable body and raised again to pick it up. It is likewise possible for the loading surface of a truck to be adjusted to the level of a loading ramp by lowering or increasing the bellows pressure at the rear axle to facilitate loading and unloading. In the case of air-sprung buses, the vehicle body on the nearside can be lowered by releasing the compressed air from the spring bellows on the nearside to make it easier for the passengers to get in and out; and it can then be raised again by filling the spring bellows.
The fundamental construction of an air suspension system of the general type under consideration is known from DE 198 33 491 C2 and DE 100 04 880 A1, for example.
The air suspension system described in DE 198 33 491 C2 has a plurality of spring bellows, which can be connected to a main pressure line and blocked off with respect thereto via connecting lines, each provided with a level control valve. The level control valves are each designed as 2/2-way solenoid switching valves, which, in a first position (rest position), are closed and, in a second position (actuated position), are open. The main pressure line can be supplied with air via a supply line provided with a compressor, an air dryer and a check valve, and can be vented via a vent line that branches off between the compressor and the air dryer and is provided with a discharge valve. The discharge valve is designed as a pressure-controlled 2/2-way switching valve, which, in a first position (rest position), is closed and, in a second position (actuated position), is open. The pilot control valve assigned to the discharge valve is designed as a 3/2-way solenoid switching valve, which, in a first position (rest position), connects the control line leading to the discharge valve to the environment and, in a second position (actuated position), connects it to the main pressure line. In a first embodiment of this known air suspension system, a throttle valve designed as a pressure-controlled 2/2-way switching valve is arranged in a line segment parallel to the check valve, which throttle valve is closed in a first position (rest position) and open in a second position (actuated position) with a throttling cross-sectional area, and the pneumatic control input of which is connected to the control line of the discharge valve. The throttle valve is thus opened by the pilot control valve when air is being supplied to the main pressure line, as is the discharge valve, wherein the throttling cross-sectional area limits the air mass flow and expands it ahead of the air dryer, thereby increasing moisture absorption by the compressed air from the air dryer and thus improving the regeneration thereof. In a second embodiment of this known air suspension system, the discharge valve and the throttle valve are combined in a common pressure-controlled 4/2-way switching valve.
The air suspension system according to DE 100 04 880 A1 differs from the foregoing prior art in that a check valve is arranged between the compressor and the air dryer and, instead of the check valve and of the throttle valve connected in parallel, a restrictor is arranged after the dryer in the supply line in the direction of air supply. Moreover, the discharge valve now has a pressure limiting function and a check valve, which is activated in the second position (actuated position). The air suspension system according to DE 100 04 880 A1 also has a pressure reservoir, which can be connected to the main pressure line and blocked off with respect thereto by means of a connecting line provided with a reservoir valve. In a first embodiment of this known air suspension system, a high-pressure discharge valve is additionally provided, the valve being designed as a 2/2-way solenoid switching valve, by means of which compressed air can be discharged into the environment from the main pressure line when required while bypassing the air dryer. In a second embodiment of this known air suspension system, a throttle valve with a controllable throttling cross-sectional area, by means of which the air mass flow flowing out into the environment when spring bellows are vented can be limited and hence the lowering speed of the vehicle body can be controlled, e.g., at one vehicle axle or on one vehicle side, is arranged after the discharge valve in the venting direction.
DE 42 43 577 B4, in contrast, describes an air suspension system of a motor vehicle in which a first control valve, which is designed as a 3/2-way solenoid switching valve and by means of which a plurality of connecting lines, each provided with a level control valve and leading to the spring bellows of an associated air spring, can be connected to a pressure source, such as a pressure reservoir, or a pressure sink, e.g., the environment, has a second control valve designed as a 2/2-way solenoid switching valve arranged after it in the direction of air supply. In a first position (rest position), this second control valve is open without throttling and, in a second position (actuated position), it is open with a throttling cross-sectional area. By actuating the second control valve, this known air suspension system can thus be switched between rapid air admission to and venting of the spring bellows and slow air admission to and venting of the spring bellows. However, the throttle of the second control valve can only be used for a certain number of spring bellows, i.e., for slow air admission to and venting of two or four spring bellows for example.
Finally, DE 102 23 405 B4 discloses an air suspension system of a motor vehicle, which corresponds substantially to that described in DE 198 33 491 C2 but in which, as in the air suspension system according to DE 100 04 880 A1, a pressure reservoir is provided that can be connected to the main pressure line and blocked off with respect thereto via a connecting line provided with a reservoir valve. A first embodiment of this known air suspension system differs therefrom in that the discharge valve is designed as a 2/2-way solenoid switching valve and a throttle valve with a controllable throttling cross-sectional area is arranged in the line segment parallel to the restrictor instead of a switching valve provided in one position with a constant throttling cross section. By virtue of the limited adjustment of the throttling cross-sectional area that is possible, the air mass flow flowing in or out via the air dryer when supplying air to and venting air from spring bellows can be regulated, and hence the raising and lowering speed of the vehicle body can be controlled zone by zone, e.g., at the relevant vehicle axle or the vehicle side. However, a throttle valve with a controllable throttling cross-sectional area is a complex component, the production of which is an involved process and which is correspondingly expensive and fault prone.
A problem with conventional air suspension systems is inadequate control or variation of the air mass flow when supplying air to and venting air from the spring bellows, and hence of the raising and lowering speed of the vehicle body. Whereas only relatively low air mass flows are required during the level control function and to compensate for leakage losses, relatively large air mass flows must be directed into the relevant spring bellows or released therefrom to lower and raise the vehicle body quickly. With the air suspension systems known hitherto, this is possible either only to an inadequate extent and in conjunction with functional disadvantages or functional limitations or only with high expenditure in terms of equipment.